The present invention relates to a scanning optical system which is employed in, for example, a laser beam printer.
In a scanning optical system for a laser beam printer, a laser beam emitted by a laser diode is deflected by a polygonal mirror to scan within a predetermined angular range. The scanning beam passes through an imaging optical system which converges the deflected laser beam to form a scanning beam spot onto a surface to be scanned, for example, a photoconductive surface. As the polygonal mirror rotates, the beam spot moves on the photoconductive surface. By ON/OFF modulating the beam spot as it moves, an electrostatic latent image is formed on the photoconductive surface.
Hereinafter, a direction, on the photoconductive surface, in which the beam spot moves as the polygonal mirror rotates is referred to as a main scanning direction, and a direction perpendicular to the main scanning direction, on the photoconductive surface, is referred to as an auxiliary scanning direction.
Further, shape and direction of power of each optical element is described with reference to directions on the photoconductive surface. Further, a plane perpendicular to a rotation axis of the polygonal mirror and including an optical axis of a scanning lens in the imaging optical system is defined as a main scanning plane.
A multi-beam scanning optical system and a tandem type scanning optical system are known in the art, as well as the above described scanning optical system in which a single laser beam is used. In the multi-beam scanning optical system, a plurality of scanning lines are formed simultaneously on a photoconductive drum. In the tandem type scanning optical system, a plurality of scanning lines are formed on a plurality of photoconductive drums, respectively.
Sometimes, a multi-beam scanning optical system or the tandem type scanning optical system is configured such that a plurality of beams are deflected simultaneously by a single polygonal mirror. If the plurality of beams are respectively inclined in the auxiliary scanning direction, and are incident on substantially the same point on the polygonal mirror, the thickness of the polygonal mirror can be reduced, which reduces a manufacturing cost of the polygonal mirror.
However, if each laser beam is incident on the polygonal mirror as inclined in the auxiliary scanning direction, i.e., as inclined with respect to a plane perpendicular to a rotational axis of the polygonal mirror, a bow occurs, that is, a scanning line, which is defined as a locus of a beam on a surface to be scanned, curves.
Further, if the scanning optical system is configured such that each laser beam emitted by the laser source is incident on the polygonal mirror from the outside of a predetermined scanning range in the main scanning direction, a change of an intersection (i.e., a deflection position) between a reflection surface of the polygonal mirror and each laser beam becomes asymmetrical with respect to an optical axis of the scanning lens, because the rotational axis of the polygonal mirror is not located on the optical axis. As a result, a scanning line is inclined with respect to the main scanning direction.
If the above described two problems occur simultaneously, a curve of the scanning line become asymmetrical with respect to a center position of the scanning line.
Since an angle of the inclination of the scanning line with respect to the main scanning direction varies according to an incident angle of the laser beam with respect to the reflection surface in the auxiliary scanning direction, it is very difficult to match all scanning lines with respect to each other. If the plurality of scanning lines corresponding to the plurality of laser beams do not coincide with respect to each other, color drift may appear in a printed image, that is, printing quality is badly affected.